1. Field of the Invention
The invention relates to the field of telephony networks, particularly those employing separate signalling channels.
2. Prior Art
In many applications there is a need for a distributed, computerized branch exchange (CBX) for facilities which are remotely located from one another. Ideally, a caller at one such facility should be able to directly dial an extension at another facility. This, of course, can most easily be accomplished by having direct or dedicated lines between these facilities. Such lines (including lease lines) are quite expensive and sometimes unreliable. One problem well recognized is the poor utilization of these lines resulting from incomplete calls, for example, the placing of a call to a remotely located busy station. Often, companies are forced to rely upon calls placed over the public voice network, which can be costly without providing adequate service.
Common channel interoffice signalling (CCIS) is used to provide better utilization (faster call setup and alternative routing) of voice links between central office. Signalling and supervisory data are transmitted over a separate data link between the offices as the call voice path is set up. When it is determined from the data link, by way of example, that a voice path can be set up, then a call is established over the voice link. While CCIS may provide increased efficiency between central offices, it does not provide a satisfactory solution or many of the features desirable for interconnecting remotely located CBXs. Tariffs are emerging which make switched common carrier facilities less expensive than dedicated private lines--used in place of an augmentation to private lines.
A CBX typically passes answer end call completion supervision over to a common carrier facility selected at the originating end (SPINT, EXECUNET, WATS, and so forth). The called end typically may be direct-inward-dialing, direct inward system accessed (DISA) or to an attendant. If the called party on a CBX to CBX intra company call is busy the CBX user still has to pay the common carrier for a completed call on either the attendant answered or DISA call. The CBX had to answer the call in these situations to get additional calling information on the called number. Direct-inward-dialing avoids this charge but this feature is not widely offered by different common carrier and those which do offer it charge accordingly. What is needed is a separate answer supervision path between end-point CBX. This patent provides such an invention. In this manner end-to-end answer supervision can be maintained in case of call blockage within the common carriers. Translation of common carrier dialing information (e.g. a specific incoming trunk at the called (CBX) can be associated with a desired extension at that CBX by information exchanged between CBX during initial call setup via the data path. Alternatively, resource availability at the two CBXs can be combined to achieve economies of scale. For example, the called CBX may have less expensive facilities available such as outwats trunks. In this case, the call would be reversed by the CBX to CBX interaction on a data link and the called CBX could originate the voice links to the voice switched common carriers. Capabilities such as those described will be increasingly important to the CBX end user as the proliferation of common carriers with a variety of tariff facilities of various service qualities as regards blockage continues. It is impractical, if not impossible, to provide a dedicated data link or switched data links between remotely located CBXs as can be done between central offices. (As the number of connected CBXs increases, the number of data links needed to fully interconnect them increases at a rapid rate. For example, 10 CBXs would require 45 links, assuming only direct links between CBXs are used). Moreover, while efficient utilization of a dedicated data link or the equivalent between central offices is possible, only poor utilization can be expected from a data link interconnecting remotely located CBXs. For a general discussion of CCIS and its use for a callback feature see: "CCIS: Signalling the Future of Stored Program Control", Telephony, May 7, 1979; and U.S. Pat. No. 4,166,929.
The present invention provides a solution to the long standing need for an efficient interconnecting system between remotely located CBXs. A "virtual network" is provided by taking advantage of the relatively low cost and high efficiency of the "virtual circuits" for transmitting data provided by the value-added networks or packet networks currently available throughout much of the United States and other parts of the world. These packet networks are used for transmitting control signals and the like between the CBXs; voice links are established over the voice Public Switched network (PSN) or comparable voice networks. ("Virtual networks" as used in this application means at least one virtual circuit used in conjunction with another network such as the voice Public Switched network.)
Numerous advantages of the invented system are described in the specification. Among the advantages is that the system is exceptionally suitable for dialing with multiple (voice) carriers. Since the system permits end point answer supervision, it allows unsuccessful calls to be retried using different carriers. Incoming calls are directly routed (without an operator) to the called station. Thus, the described system provides a distributed CBX with automatic station-to-station capability.